Why does importing workflow from a jwb file create duplicate screenshots and custom fields?

Question

JIRA

I ran into a problem when importing workflow from jwb files. When importing, screens, custom fields dependencies are created. Everything would be fine, but when you import several workflows, each of them creates its own set of custom fields and screens. In fact, the same screen and the same custom fields are used for several workflows. How to properly clean the system from unnecessary custom fields and screenshots?